Generally, shoes serve to protect a person's feet. Thus, when a person runs or walks with shoes on, the shoes isolate the feet from the outside, in addition to protecting the feet.
If a person weighing about 60 kg walks an average of 6,000 steps per day, a load of about 432 tons is consequently applied to the feet. This is equal to a load exceeding the weight (about 400 tons) of a typical jumbo jet used for international flights.
As such, the simplest method of developing a shoe that absorbs shocks transmitted to the sole of the foot is to reduce the hardness of the shoe. Provided that cushioning is simply defined as reducing a maximum force, good cushioning can be defined as absorbing shocks in order to prevent skeletal muscles from being strained. However, since a propulsive force as well as the cushioning is an important factor in exercise performance, it is undesirable that a given force be completely absorbed by a cushioning system. As a result, this is a complex problem, and it is not easy to define optimum shoe hardness for the responsive shock absorption of the skeletal muscles in the case where repeated shocks occur. This is why information about the magnitude of force should be acquired and considered.
Based on the result of analyzing a repulsive force against the ground during running using a pressure plate, it can be seen that the magnitude of a vertical force between the ground and the foot is two to three times as large as a person's weight. Assuming that the contact numbers of both feet and the ground during running are 1,000 times per 100 m, when the force that is two to three times as large as the weight acts as repeated load and shocks are not sufficiently absorbed, a joint may suffer from a degenerative change, and backache may also be caused. Thus, it is preferable to select running shoes with a good cushioning effect that guarantees excellent shock absorbing performance.
FIGS. 1(a) to 1(c) schematically illustrate a weight impact region on the sole of the foot during walking. In FIG. 1(a), for an initial 20 to 30 ins when a rear region of the foot comes into contact with the ground, an impact force is larger than the weight, that is, the foot is subjected to shocks that are 2.2 times as large as the weight. Thereafter, as shown in FIG. 1(b), in the state where the sole of the foot entirely comes into contact with the ground, the weight is larger than supporting force. Further, as shown in FIG. 1(c), as for force generated for 100 ins when a front region of the foot comes into contact with the ground to perform running again and then is taken off from the ground, the propulsive force is larger than the weight. That is, it is reported that the sole of the foot is subjected to load that is 2.8 times as large as the weight.
As such, it can be seen that the shocks applied to the sole of the foot during walking or running are larger than previously thought.
Thus, in order to mitigate these shocks, various types of shock absorbers, for example, airbags or sponge foam, are utilized. In the case of using a shock absorber, it is useful for absorbing shocks to some extent. However, the better the shock absorbing force is, the lower the repulsive force against the ground is. Thereby, when a user walks or runs for a lengthy period of time, the feeling of fatigue is further increased.
That is, in the structure of the shoe, the absorption of shocks that should be reduced when the foot is comes into contact with the ground and the repulsive elasticity that should be increased when the foot is taken off of the ground are symmetric to each other. When the shoe satisfies either of the shock absorption or the repulsive elasticity, the remaining one is undesirably sacrificed.
Further, the shoe is problematic in that the shock absorber may be broken or damaged by large shocks acting on the sole of the foot.
Furthermore, when the center of a user's weight leans to the front, shocks are not absorbed.
In order to overcome the above problems, the inventor(s) of the present invention filed KR Patent No. 10-1166466, which is entitled “shoe with shock absorbing means having high durability”.
However, the cited invention is problematic in that the durability of a coupling portion between an upper shock absorbing portion and a lower shock absorbing portion is poor.
Further, it is problematic in that shocks transmitted to the shock absorbing means are mitigated at a center point rather than at opposite sides, with the result that the shoe is poor in balance performance and stability.